The largest, most comprehensive study of its kind to date has found that women who have undergone tubal sterilization are at no greater risk for menstrual abnormalities than are women who have not had the procedure, settling a debate within the medical community.

The study authors also found that women who underwent the procedure appeared to have fewer menstrual abnormalities than did women who were unsterilized, but quickly cautioned that they regarded this secondary finding as most probably a chance occurrence. The researchers stressed that there is no known biological reason why tubal sterilization should reduce the risk of menstrual abnormalities.

The study, a collaboration between National Institute of Child Health and Human
Development (NICHD) and the Centers for Disease Control and Prevention (CDC), appears in the December 7 New England Journal of Medicine.

"These results put to rest any last lingering concerns about the safety of tubal sterilization," said Duane Alexander, M.D., Director of the NICHD.

Herbert Peterson, M.D., of CDC's reproductive health program and his co-authors arrived at their findings after analyzing information from the U.S. Collaborative Review of Sterilization (CREST) study, a large, multicenter study begun in 1978 to evaluate the long-term safety and effectiveness of tubal sterilization. In tubal sterilization  commonly referred to as tubal ligation  a woman's fallopian tubes are blocked by any one of a variety of methods, to prevent sperm and egg from coming in contact with each other. The researchers compared 9514 women who had undergone tubal sterilization to 573 women whose husbands had been sterilized by vasectomy.

Each year for five years, both groups of women were interviewed by telephone. The women were asked about the length of their menstrual cycles, bleeding or spotting between periods, whether their cycles were irregular, the amount of pain they experienced during menstruation, and the amount of bleeding they experienced each month.

Compared to the women who were not sterilized, women who had undergone tubal sterilization were no more likely to report changes in bleeding between menstrual periods or in the length of periods. However, compared to the non-sterilized women, the sterilized women reported decreases in the number of days of bleeding, the amount of bleeding, and in menstrual pain. The sterilized women also were more likely to report having irregularly occurring periods. The study authors believe the differences between the two groups of women were either due to chance or to unknown differences between the two groups of women.

"We know of no biologic explanation for these changes, most of which were beneficial, in sterilized women," they wrote.

The authors noted that they could not rule out the possibility of menstrual abnormalities occurring more than 5 years after sterilization. Although they had information on the sterilized women for up to 14 years after the women had undergone the procedure, the researchers were able to obtain information on the unsterilized women for only five years. The researchers added, however, that if changes in menstrual patterns have not developed by five years after the procedure, then it is unlikely that they would develop after that time.

In the New England Journal of Medicine article, the authors also explored other explanations for the differences between the two groups. For example, some researchers theorized that changes in blood flow to the ovaries, brought about by the sterilization procedure, could alter the ovaries' ability to produce hormones. Possibly, such altered hormone levels could result in fewer menstrual abnormalities for the sterilized women. The authors discounted this possibility, however, noting previous studies of sterilized women did not identify any abnormalities in ovarian function.

The researchers also noted that many of the women in the sterilized group had been taking oral contraceptives before the procedure. Conceivably, discontinuing oral contraceptives might result in hormonal changes affecting menstruation.

"Our data argue against any syndrome of menstrual problems after sterilization," the researchers concluded.